The invention relates to an apparatus for isolating an evaporative solvent from a polymer solution to recover a concentrated solution. In particular, the invention relates to an apparatus for isolating hydrous acetic acid from an acetyl cellulose solution that has passed through acetylation and aging steps, to thereby obtain a solution having a concentration suitable for feeding it to a subsequent precipitation step.
Generally, the more concentrated is a solution that contains a polymer as solute, the higher is its viscosity and the smaller its heat transfer efficiency. Therefore, evaporation under forced agitation or in the form of a thin film is conventionally used to perform concentration without reducing its efficiency. For this purpose, a uniaxial bent extruder, biaxial bent extruder and talling film evaporator, and the like have been used. Usually, these devices use the surface of a casing wall as a heating surface, and in most cases, the gap between the heating surface and the revolving parts, namely, the clearance between the surface of the casing wall and the farthest end of the agitating blades, is held to a minimum and each agitating blade is provided with scraper at its end.
With this construction, a large apparatus cannot be manufactured because of limited precision as to the symmetry of machined parts around the revolving axis. For instance, an apparatus having a heat transfer surface larger than 10 m.sup.2 is very difficult to manufacture. Furthermore, the deterioration of the polymer in the solution due to local overheating can be effectively prevented by evaporating the solvent under vacuum, but an apparatus of an agitator type with an airtight construction is complex and accompanied by a high production cost. An acetyl cellulose solution just after aging contains not only acetyl cellulose but also water-soluble inorganic salts as solute. During the concentration step, these salts form crystals which tend to be rubbed between the agitating blades and the surface of the casing wall. The crystal particles wear the blades and surface of the casing wall, and they also contaminate the acetyl cellulose solution.
An acetyl cellulose solution after aging is composed of a solvent made for acetic acid and water and a solute made for acetyl cellulose and salts supplied or formed during aging. Containing about 20% of acetyl cellulose, the solution in this state is generally referred to as a dope. The acetyl cellulose in the dope is recovered by introducing the dope into water (non-solvent) to form a precipitate. If the dope concentration is increased to 25-30% of acetyl cellulose before it is sent to the precipitation step, less non-solvent need be used in the precipitation step and the amount of heat required for solvent recovery throughout the process steps is reduced greatly. But in the prior art technology, there is no apparatus suitable for concentration of the acetyl cellulose dope prior to the precipitation step.